When people read books, it's usually a book of a couple hundred pages. Not short stories or poems.
When they watch a movie, they usually watch a feature length film. Not a festival short film.
When I was in college, I got an minor in English, mostly taking classes with short form writing (poetry, short stories, and poems) because the thought of reading a pile of novels in a single semester seemed unachievable, and it least if I didn't do the reading back in the dorms I could still contribute to class discussion by reading the poem or short story real quick.
And while I can only imagine how hard it is to get a novel published, the thought of getting a series of short stories published (a la, James Franco) or a poetry anthology seems increasingly challenging because the market for such genres seems so limited.
The short film seemed doomed to the same fate. The only time I really think of short films is during the Oscars and I think about how I don't care for the category.
And here's why I think Short form art is not popular:
If I'm reading a book or watching a movie. The primary reason is to be entertained. It may take a couple dozen pages or a few minutes into a film to get hooked, but if it's good - I'll be grabbed.
If it's a short story, I'll read, I'll enjoy, but when it's done, that initial push that gets me to read the next work in the anthology or collection is a speed bump I don't experience in the same way with chapters in fiction.
I believe this is the same with a short film as well.
Similarly, if you're reading poetry or engaging one of these short art forms, they usually take the opportunity of the short form to be artistic, shocking, or have the intentions of being conversation inducing.
This works great in a college class where you all read the same poem, you all discuss, and you all fall in love with the text. But in isolation, the thought of reading poetry by myself with no outlet for discussion sounds, well...uninspiring.
It's a pity. But I think the lack of communal investment in a medium that strives for art over entertainment makes short, well...not popular. And certainly not marketable.
Why news, journalism, and even TV shows are exceptions:
Where films, fiction, and poetry are typically mediums for art and entertainment. Journalism is about passing information. I can read an interesting news story or pick up a magazine and not feel the need to pass the information along to others. I can feel informed, and given the opportunity to pass the time.
In fact, in this medium, the idea of reading a long article in one of the prestige journals, sound dreadful, because I'm not that interested in most of the topics that might be covered.
TV is a little different. TV has found a way to be a huge brain-fry of entertainment. No need to discuss it. No need to get the motivation to watch it...it's just there, playing whether I'm there or not. Sure some shows are artful, but generally TV has found a way to abandon art and focus on marketability and sustainable entertainment...so people watch it, even if it's in a 30-minute format.
Just some thoughts on short.